1. Field of the Invention
This invention relates to a program controlled telephone switching system and, in particular, to a switching system of the time division type. More particularly, the invention relates to facilities for reducing the real time requirements of the system processor or controller.
2. Description of the Prior Art
It is known in prior art program controlled switching systems to use a single processor comprising a control unit and associated bulk memory to perform the system work functions. The work of the processor in such systems may generally be divided into several different functions. The first is the scanning of line and trunk circuits or ports to detect hook state signals originating in or received by those circuits. The processor uses these signals to derive call status information as well as to identify dialed digits. The second function performed by the processor is the call processing work that must be executed to control the establishment and release of call connections. A third function relates to the maintenance and administrative programs that must be executed periodically to assure the reliable system operation.
The use of a single processor is satisfactory so long as the processor throughput is sufficient to accommodate all of the tasks it is required to perform. However, as the system increases in size or call activity, the demands on the processor may equal or exceed the processor's capability. Arrangements must then be made either to increase the processor capacity or to reduce its real time requirements.
It is known in the prior art to provide auxiliary facilities which reduce the real time requirements on the processor by performing some of the more routine and time consuming system chores. Examples of such chores are line scanning and the analysis of scanning information to detect certain call states such as switchhook flash, call abandon, dial pulses, etc. The following three U.S. Pat. Nos. disclose prior art arrangements of this type:
3,408,628; Brass et al; Oct. 29, 1968; 3,627,954; Quinn; Dec. 14, 1971; 3,629,851; Ulrich; Dec. 21, 1971.
Brass discloses a stored program controlled switching system having an auxiliary processor (termed a signal processor) which performs some of the more repetitive and time consuming system tasks such as the scanning of line and trunk circuits. The signal processor interacts with the main processor by means of shared memory common to both elements.
Quinn discloses a stored program controlled switching system having an auxiliary processor for performing repetitive and time consuming system tasks such as detecting the current state of lines and trunks, controlling the registration of dialed digits, and controlling the outpulsing of digits to distant offices.
Ulrich discloses a stored program controlled switching system having a main processor and an auxiliary processor. The auxiliary processor performs repetitive chores such as scanning lines and trunks to detect call state signals that may represent service requests. Upon the detection of such a signal, the auxiliary processor notifies the main processor via a shared memory that a specified task must now be performed for the requesting line or trunk.
The auxiliary processor facilities disclosed in the above cited prior art are satisfactory in that the provison of an auxiliary processor in any system obviously reduces the work load on the main processor and thereby increases the system's call serving capability. However, these auxiliary processor facilities are complex and costly, they involve a significant modification to the original system and, to a degree, merely amount to the use of a second system processor. Therefore, even though these arrangements are satisfactory for the systems for which they were designed, the cost penalty associated with them precludes their use in systems in which cost considerations are paramount.